The invention relates to a bottom resist for the two-layer technique.
In microelectronics, resists are used for planarization and for optical decoupling of substrates. These resists must be able to be applied to the substrate by a spin coating technique and cured by appropriate techniques. Furthermore, it must be ensured that the resists are insoluble when overcoated with a photoresist as is used for exposures in the deep UV (248 nm, 193 nm, 157 nm or 126 nm). The photoresists used with preference in this wavelength range are based on the principle known as chemical amplification (cf., for example, Jpn. J. Appl. Phys., Vol. 31 (1992), Pt. 1, No. 12B, pages 4273 to 4282). A disadvantage here, however, is that the resists react with great sensitivity to the basicity or acidity of the underlying material (cf., for example, TI Technical Journal, Vol. 14, No. 3, May-June 1997, pages 17 to 23).
In the single-layer technique, resists which are resistant to substrate etching are used on thin antireflection layers possessing a very low resistance to substrate etching in a halogen plasma. These materials are intended specifically to cause superimposition of optical reflections and to generate an interference pattern which following development leads to vertical structures in the top resist. Although these thin materials ( less than 100 nm) are adapted in terms of acidity to chemically amplified resists, their etch resistance makes them unsuitable for applications in the conventional two-layer technique (thin film imaging).
So-called bottom resists adapted to the two-layer technique with chemically amplified photoresists have not been disclosed to date. A so-called CARL resist (CARLxe2x80x94Chemical Amplification of Resist Lines) used in the I-line technology (exposure at 365 nm) is a diazonaphthoquinone-based resist which is baked at a temperature of about 300xc2x0 C. for several minutes. Owing to the high proportion of basic constituents, however, this material cannot be used as an underlayer for chemically reinforced resists. Following development, indeed, unacceptable residual layers remain in the exposed regions, or there is what is known as footing; i.e., the developed structures are substantially broader at the interface with the underlying bottom resist and, consequently, the desired vertical profiles are not produced.
Similar comments apply to negative resist systems which operate with crosslinking agents containing basic groups, such as nitrogen atoms. Negative resists of this kind, furthermore, are only insoluble in aqueous-alkaline developers, but lack sufficient insolubility and swelling resistance in the solvent of the top resist that is to be applied.
It is accordingly an object of the invention to provide a bottom resist intended for the two-layer technique that overcomes the above-mentioned disadvantages of the prior art and affords both a well-adapted surface acidity in order to be compatible with chemically reinforced photoresists and also a high level of stability in a halogen plasma as used for the structuring of the underlying substrate, such as silicon.
With the foregoing and other objects in view there is provided, in accordance with the invention, a bottom resist comprising the following components:
a phenolic base polymer,
a thermoactive compound which releases a sulfonic acid above a temperature of 100xc2x0 C., and
a solvent.
The proportion of base polymer is advantageously from 8 to 40% by mass, preferably from 8 to 15% by mass, and the proportion of thermoactive compound is advantageously from 0.005 to 4% by mass, preferably from 0.02 to 0.1% by mass, the remainder (to 100% by mass) comprising solvent.
The bottom resist according to the invention is tailored to the chemically reinforced two-layer technique which uses a thin photosensitive resist over a thick planarizing layer which is resistant to substrate etching. With this bottom resist, the crosslinking reaction is purely thermal, and takes place at relatively low temperatures ( greater than 100xc2x0 C.) and within a short time ( less than 90 s). The shelf life of the bottom resist is for example at least six months at a temperature, of 40xc2x0 C. The base polymer of the resist includes a high proportion of aromatic structures, thereby imparting a high level of etch stability.
The bottom resist according to the invention can advantageously further comprise one or more of the following components:
Crosslinking-active compound (concentration: from 0.005 to 4% by mass):
This compound accelerates the crosslinking or curing (of the bottom resist layer). For this purpose it is preferred to use compounds having at least two hydroxymethyl groups, such as 2,6-hydroxymethyl-p-cresol.
Basic compound (concentration: from 0.005 to 4% by mass):
Using this compound it is possible to reduce further the acidity of the surface of the resist material (after curing). For this purpose it is preferred to use pyrrolidone derivatives and piperidine derivatives, an example being N-methylpyrrolidone.
Photoactive compound thermally stable up to 235xc2x0 C., which on exposure to radiation releases a sulfonic acid (concentration: from 0.005 to 4% by mass):
This compound acts to generate an acid at the interface between bottom resist and top resist upon subsequent exposure (in the two-layer technique). For this purpose it is preferred to use triarylsulfonium salts.
Dye (concentration: from 0.025 to 12% by mass):
This makes it possible to adjust the absorption and the refractive index of the resist material. The dye has an xcex5 value  greater than 10,000 at the respective exposure wavelength. For this purpose it is preferred to use hydroxymethylanthracene (anthracenemethanol).
Additives (amount: from 0.0001 to 0.04% by mass):
These can in particular be flow control agents (leveling agents). An additive of this kind enhances the planarizing properties of the resist.
The adaptation of the bottom resist to a top resist is ensured for the following reasons:
1. The acidity can be adjusted by means of suitable acid generators and/or addition of bases as required. Acid generators, also referred to as thermal acid generators, comprise, in particular, compounds which undergo thermal decomposition at a temperature between 100 and 235xc2x0 C. In principle it is possible to use a relatively large number of conventional compounds, although these compounds must be modified chemically so as to reduce the thermal stability and so that the acid generated has the desired mobility. Furthermore, the solubility must be borne in mind and it must also be ensured that the crystallization tendency (formation of particles) is as low as possible. This can be brought about by means of various chemical substituents, such as branched alkyl groups, which function as solubilizers, and by means of suitable parent structures.
2. The optical parameters can be adjusted by adding appropriate dyes. What is important in this context is a very high absorption at the exposure wavelength and a very low volatility during processing. Ideally, the dye is also able at the same time to adopt the function of a crosslinking-active compound or of a basic compound or of a photoactive, thermally stable compound, or two or more of these functions together. An appropriate dye might even serve as the thermal active compound itself.
3. The curing of the bottom resist is carried out such that no swelling effects occur when the top resist is applied. This is achieved firstly by the base polymer having a large number of chemically reactive groups and secondly by minimizing the presence of very few fractions of uncombined, free constituents. The latter is the case when after the crossinking reaction only a very small fraction of low molecular compounds remains.
The thermoactive compound (thermal acid generator) is preferably selected from one of the classes of compounds defined by the structures shown below, in each of which Xxe2x88x92 represents a sulfonate anion and a sulfonate radical:
onium salts of structure R2I+Xxe2x88x92 (1):
The radicals R independently of one another each denote C1 to C12 alkyl or (C6 to C14 aryl)xe2x80x94COxe2x80x94CH2xe2x80x94. In radicals R containing aromatic groups, these groups can be substituted by C1 to C12 alkyl groups, by C6 to C14 aryl groups, or by one or more alkoxy, hydroxy, halogen or nitro groups.
Onium salts of structure R3S+Xxe2x88x92 (2) or R2ArS+Xxe2x88x92 (3):
The radicals R independently of one another each denote C1 to C12 alkyl or (C6 to C14 aryl)xe2x80x94COxe2x80x94CH2xe2x80x94 or two radicals R together form a tetramethylene group and Ar denotes C6 to C14 aryl, of which one or two carbon atoms can be replaced by O, N or S. The aromatic radicals Ar can be substituted by C1 to C12 alkyl groups, by C6 to C14 aryl groups, or by one or more alkoxy, hydroxy, halogen or nitro groups. In onium salts of structure (2), R can also denote benzyl (C6H5xe2x80x94CH2xe2x80x94), in which case the aromatic component can be unsubstituted or substituted by an alkoxy, hydroxy, halogen or nitro group.
Onium salts of structure RI+ArI+R2Xxe2x88x92 (4) or R2S+ArS+R22Xxe2x88x92 (5):
The radicals R independently of one another each denote C1 to C12 alkyl or (C6 to C14 aryl)xe2x80x94COxe2x80x94CH2xe2x80x94 or two adjacent radicals R together form a tetramethylene group and Ar denotes C6 to C14 aryl, of which one or two carbon atoms can be replaced by O, N or S. The aromatic radicals Ar can be substituted by C1 to C12 alkyl groups, by C6 to C14 aryl groups, or by one or more alkoxy, hydroxy, halogen or nitro groups.
Benzylsulfonic esters of structure Arxe2x80x94CH2xe2x80x94X:
Ar here denotes C6 to C14 aryl, in which one or two carbon atoms can be replaced by O, N or S. The aromatic radical can be substituted by one or more alkyl, alkoxy, hydroxy, halogen, cyano or nitro groups.
The sulfonate anions Xxe2x88x92 and the sulfonate radical X are selected preferably from one of the following groups:
linear, branched or cyclic C1 to C12 alkylsulfonate group, for example, a hexadecylsulfonate, cyclohexanesulfonate or camphorsulfonate group;
mono-, poly- or perhalogenated C1 to C12 alkylsulfonate group, for example, a trifluoromethane sulfonate (triflate) or nonafluorobutanesulfonate group;
mono- or polyhalogenated C6 to C14 arylsulfonate group, for example, a pentafluorobenzenesulfonate group;
C6 to C14 arylsulfonate group substituted one or more times by an electron acceptor, for example a dinitrobenzene sulfonate group;
C6 to C14 arylsulfonate group substituted one or more times by a C1 to C4 alkyl radical, for example, a p-toluenesulfonate group.
The following thermoactive compounds may be mentioned by way of example:
Onium salts of structure (1):
Dimethyliodonium p-toluenesulfonate, diethyliodonium nonafluorobutanesulfonate, methyl(phenylcarboxymethyl)iodonium p-toluenesulfonate and methyl(anthrylcarboxymethyl)iodonium nonafluorobutanesulfonate.
Onium salts of structure (2) and (3):
Trimethylsulfonium camphorsulfonate, dimethyl(phenylcarboxymethyl)sulfonium p-toluenesulfonate, benzylthiolanium nonafluorobutanesulfonate, p-methoxybenzylthiolanium triflate, p-nitrobenzylthiolanium triflate, anthryldimethylsulfonium triflate and phenothiazinyldimethylsulfonium triflate.
Onium salts of structure (4) and (5):
In this case the aromatic radical Ar can, for example, be bisphenyl or anthryl, such as in bis(methyliodonium)-9,10-anthracene bistriflate.
Benzylsulfonic esters:
Benzyl-p-toluenesulfonate, p-methoxybenzylcamphorsulfonate, p-cyanobenzylcamphorsulfonate, o-nitrobenzyltoluenesulfonate and phenothiazinylmethyl-p-toluenesulfonate.
In the bottom resist according to the invention, the phenolic base polymer is preferably a novolak or a poly-p-hydroxystyrene. Examples of further suitable base polymers are polyimides and polybenzoxazoles each having phenolic OH groups. In general, at least every third monomer unit in the base polymer, on average, has an OH group.
The solvent is preferably propylene glycol monomethyl ether acetate (methoxypropyl acetate), cyclopentanone, cyclohexanone, xcex3-butyrolactone, ethyl lactate, or a mixture of at least two of these compounds. In general, however, all common photoresist solvents can be used.
The bottom resists according to the invention are combined with selected top resists, especially with chemically reinforced CARL resists (cf. Proc. SPIE, 1998, Vol. 3333, pages 154 to 164). In this context the exposure wavelength also plays an important part, since the optical parameters, such as the real and imaginary components of the refractive index, are dependent on the wavelength.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is described herein as embodied in a bottom resist, it is nevertheless not intended to be limited to the details given, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the following examples.